Recent progress in TiO2–biochar-based photocatalysts for water contaminants treatment: strategies to improve photocatalytic performance

Toxic organic pollutants in wastewater have seriously damaged human health and ecosystems. Photocatalytic degradation is a potential and efficient tactic for wastewater treatment. Among the entire carbon family, biochar has been developed for the adsorption of pollutants due to its large specific surface area, porous skeleton structure, and abundant surface functional groups. Hence, combining adsorption and photocatalytic decomposition, TiO2–biochar photocatalysts have received considerable attention and have been extensively studied. Owing to biochar's adsorption, more active sites and strong interactions between contaminants and photocatalysts can be achieved. The synergistic effect of biochar and TiO2 nanomaterials substantially improves the photocatalytic capacity for pollutant degradation. TiO2–biochar composites have numerous attractive properties and advantages, culminating in infinite applications. This review discusses the characteristics and preparation techniques of biochar, presents in situ and ex situ synthesis approaches of TiO2–biochar nanocomposites, explains the benefits of TiO2–biochar-based compounds for photocatalytic degradation, and emphasizes the strategies for enhancing the photocatalytic efficiency of TiO2–biochar-based photocatalysts. Finally, the main difficulties and future advancements of TiO2–biochar-based photocatalysis are highlighted. The review gives an exhaustive overview of recent progress in TiO2–biochar-based photocatalysts for organic contaminants removal and is expected to encourage the development of robust TiO2–biochar-based photocatalysts for sewage remediation and other environmentally friendly uses.


Introduction
Global concern has been sparked by the fact that water pollution endangers human life and harms ecosystems. 1,24][5][6] Several human diseases, even cancer, have been related to organic pollutants in water. 7,8Therefore, hazardous contaminants found in wastewater must be appropriately disposed of to protect human health and the environment and to guarantee safe discharge.Most organic pollutants are recalcitrant, toxic, and have weak biodegradability.Thus, they cannot be degraded absolutely via traditional treatment techniques (for example, adsorption, biodegradation, and direct burning). 9,10Developing a promising treatment technology is paramount for effectively degrading contaminants and ensuring sustainable development.The photoelectrocatalytic water splitting experiment for H 2 evolution under ultraviolet light in 1972 led to the development of photocatalysis technology. 11,12][15] The core of photocatalytic degradation of pollutants is to develop highly efficient and inexpensive photocatalysts.Titanium dioxide (TiO 2 ) nanoparticles are now regarded as one of the best photocatalytic materials because they have unique electrical and optical properties, are chemically stable, and don't harm living things. 16However, the TiO 2 photocatalyst, which has a broad energy band gap (3.0-3.2 eV) and can only soak up radiation in the near ultraviolet (UV) zone (approximately 5% of the total sunlight spectrum), is limited to visible light applications. 17Furthermore, most single-component photocatalysts are provided in the form of tiny particles (such as nanoscale), which effectively increases the specic surface area of the material and aids in enhancing the photocatalytic effect.However, it is more noteworthy that tiny particles can also greatly enhance the intermolecular force, leading to agglomeration effects during the treatment of pollutants, which hinder the effective decomposition of pollutants.At the same time, the recycling and reuse of nanoscale catalysts also face challenges. 18To alleviate the drawbacks, integrating photocatalytic materials with substrate materials for modication has been gradually developed and utilized.Representative substrates, including carbon materials (like activated carbon, multiwalled carbon nanotube, and reduced graphene oxide), Jia Li received her B.S. in Physics from Changchun University in Jilin, China in 2010, and her M.S. and Ph.D. in condensed matter physics from Jilin University in China in 2015.She works at the School of Science at Beihua University in Jilin, China.For many years, she has been engaged in the research and development of new materials and physical properties.Her current research focuses on developing new carbon-based related materials, such as low-dimensional diamond and boroncarbon-nitrogen sandwich structures, exploring new structures and their electronic and optical properties, and expanding their applications in photocatalysis, nanoelectronic devices, and sensors.
0][21] Considering the quick recombination rate of photoinduced carriers and the low thermal stability of most substrates, carbon materials are superior in photocatalytic degradation applications. 22Carbon-TiO 2 composites have drawn growing interest as outstanding photocatalysts since carbon materials can serve as electron traps for gathering photoinduced carriers from the photocatalyst surface. 23,24At the same time, carbon doping is an efficient approach for introducing impurities into the TiO 2 lattice, which decreases the band gap and extends visible light absorption. 257][28] These characteristics are advantageous for the binding and growth of photocatalyst nanoparticles, leading to signicant loadings, robust stability, and remarkable organic waste photodegradation capabilities. 29,30Therefore, biochar is an excellent support for TiO 2 -based catalysts, and in recent years, TiO 2 -biochar-based photocatalysts have been recommended and studied widely.
The combination of TiO 2 semiconductor and biochar offers several advantages: ][37][38] (2) As a supporting material for TiO 2 photocatalysts, biochar has a stable structure and easy recovery.
(3) The porous structure and large specic surface area of biochar provide solid guarantees for the adsorption of pollutants, which can effectively increase the contact between pollutants and TiO 2 photocatalysts, shorten the carrier migration path, and improve the photocatalytic effect.
(4) Biochar helps create vacancies, acting as photoelectron traps for captured TiO 2 , thereby suppressing the annihilation of photogenerated e − -h + pairs.Concurrently, biochar enhances the heat resistance of photocatalysts, reducing their band gap and boosting the response to light sources. 395) The composite, coupled with biochar and TiO 2 semiconductor, could present unique and complementary physicochemical properties in favor of water treatment applications. 40owever, there are also many challenges associated with the combination of biochar and TiO 2 photocatalysts: (1) The preparation method of the composite photocatalysts directly affects the bonding strength between biochar and TiO 2 .The nonspecic binding of TiO 2 photocatalysts may lead to a signicant decrease in their effectiveness over time.
(2) The impurities and functional groups in biomass, inuenced by factors such as growth environment, can lead to unpredictable performance of each batch of prepared biochar.
(3) The strong biochar adsorption also hinders the recycling of composite photocatalysts.
(4) Improperly applied amounts of biochar shade the light of TiO 2 photocatalyst, affecting the generation efficiency of carriers.
It is time to review this intriguing topic to recapitulate current developments and, fundamentally, to acquire an indepth knowledge of the application of TiO 2 biochar-based materials in photocatalytic decontamination.This review will concentrate on the synthesis methods, physicochemical properties, advantages, and photocatalytic capacity of TiO 2 -biocharbased photocatalysts to remove organic effluent pollutants.Three aspects of some prospective photocatalytic efficiency enhancement strategies are highlighted.Lastly, the signicant obstacles and prospects of photocatalysts based on TiO 2 -biochar are presented.

Biomass for biochar production
Biochar is a unique and renewable carbonaceous substance derived from decomposing biomass residues in oxygendepleted conditions. 41Its porous structure and multitudinous oxygen-containing functional groups on its surface make it suitable for binding pollutant molecules. 42In addition, biochar's tting compatibility and catalytic properties make it an exceptional pollutant removal material for practical application in photocatalysis. 43

Preparation methods of biochar
It is widely accepted that biomass is the primary source of biochar's feedstock.Carbon-rich biomasses can be regenerated from agricultural and forestry byproducts, garbage, woody products, plants, animal manures, and dietary leovers. 44,45It comprises both lignocellulosic (woody products, agricultural and forestry byproducts, plants) and non-lignocellulosic products (garbage and animal leovers). 46,47Thermochemical techniques can directly convert these numerous forms of biomass into biochar materials. 48The most investigated thermochemical treatment methods are oxygen-limited pyrolysis and hydrothermal charring.
Pyrolysis is mainly divided into slow pyrolysis, fast pyrolysis, and ash pyrolysis.Among them, slow pyrolysis has been extensively adopted because of its substantial biochar yield, simple operation, and low cost. 49,50Zhang et al. 51 produced biochar by pyrolyzing leovers from a school cafeteria in oxygen-depleted conditions.The resultant biochar with an even surface was considered an appropriate supporter for attaching BiOBr.Zhang et al. 52 prepared biochar using crop straws as a precursor by pyrolysis at 600 °C for 120 min under limited oxygen conditions.Luo et al. 53 crushed poplar to obtain poplar sawdust and then heated it at 600 °C under N 2 atmosphere to obtain biochar (BC), which served as a conductive aisle in the ZnFe 2 O 4 /BC/ZnO composite to improve the transportation of photoexcited electrons.Meanwhile, pyrolysis could be used to fabricate biochar doped with other elements.For instance, Wang et al. 54 mixed urea and pine biochar at a particular mass ratio to produce N-modied biochar by pyrolyzing in a vacuum muffle furnace at 500 °C for 120 min.The heating temperature is one of the crucial points for producing biochar via pyrolysis.
According to previous studies, [55][56][57] researchers commonly set the heating temperature at 300-900 °C.Generally, the surface area and pore quantity of biochar increase as the degree of heat rises.However, pyrolyzing biomass at an excessively extreme temperature may cause the carbon skeleton to split, leading to the opposite result. 58,59Various pyrolysis procedures will produce biochar with unique physicochemical features and varying application efficacy. 60In addition, Shi et al. 61 prepared biochar with different pyrolysis temperatures, which manifested that the photocatalytic ability of biochar rose as pyrolysis temperature decreased.Therefore, an in-depth study of how heating temperature affects biochar's surface features and physicochemical characteristics is worthwhile for the further development of biochar.Overall, modulating heating rates and maintaining an exact response temperature could prove challenging in the pyrolysis process. 27he other efficient method for preparing biochar is hydrothermal carbonization (HTC).Typically, HTC is conducted at comparatively lower temperatures and more signicant pressures for converting biomass to biochar. 62Xu et al. 63 reported that N-doped carbons were generated by HTC at 200-260 °C using sewage sludge, rice husk, and cellulose as precursory biomass materials.Cai et al. 64 employed crushed peanut shells as raw biomass materials by hydrothermal reaction in the existence of iron chloride and hexamethylenediamine (HDA) to produce magnetic HDA-modied biochar.Liu et al. 65 added measured lotus powder to a nitrate solution (La(NO 3 ) 3 $6H 2 O and Fe(NO 3 ) 3 $6H 2 O were mixed with deionized water) and then carried out a hydrothermal reaction at 180 °C for 90 min.Lastly, C-doped LaFeO 3 /biochar was produced by calcining the sample.Moreover, because carbon content relates directly to ash quantity and pyrolysis temperature, biochar with the desired carbon and ash content can be produced by modulating the hydrothermal temperature. 45Compared with pyrolysis, HTC has a higher product yield (up to 88%) and lower energy consumption. 66HTC is needed for subsequent processes, such as ltration, centrifugation, and drying, to obtain the desired biochar.
Besides that, the combination of pyrolysis and the HTC process has been conventionally utilized to dope elements into biochar or form biochar-based composites.Talukdar et al. 67 successfully synthesized Ag 3 PO 4 /Fe 3 O 4 co-doped bambooderived activated biochar via two-step processes.Specically, bamboo-derived activated biochar (BAB) was generated by pyrolysis at 500 °C, followed by a wet chemical process to produce an Ag 3 PO 4 /Fe 3 O 4 co-doped BAB composite.Chen et al. 68 rst prepared micro-mesoporous carbon sheets (MMCSs) by pyrolyzing the luffa sponge in a tubular furnace, followed by KOH activation.Then, the obtained MMCSs were mixed with thiourea and dispersed into the distilled water to construct micro-mesoporous carbon sheets doped with nitrogen and sulfur via a hydrothermal reaction.
][71] The type of biomass material has the most signicant impact on biochar's xed carbon and ash contents.Biochar with a high ash content is unsuitable for capturing or eliminating contaminants due to its diminished adsorption sites and micropore surface areas. 49,724][75] Biochar's specic surface area can be increased, and its aromaticity can be altered through alkali modication. 76Besides, while metal modication might create additional catalytic active sites, pristine adsorption sites may be occupied, or porous structures may be blocked. 77Thus, the design of the doping site and the loading ratio on biochar should be deeply considered.Overall, biochar's surface properties correlate with catalytic performance, so optimizing preparation strategies for biochar catalytic application is imperative.

Adsorption over pristine biochar
The remarkable characteristic of biochar is its adsorption capacity because of its porous nature, enormous specic surface area, and adequate functional groups. 78Applications in the adsorption and decomposition of contaminants necessitate a porous material with a high surface area.The specic surface area and porosity may affect the biochar's adsorption capacity and the location of catalytically active sites, thereby determining its performance. 79More opportunities for contaminants to connect with biochar surface functional groups and photocatalysts can be obtained in the biochar with a greater surface area and porosity, accelerating adsorption and decomposition rates.Composite photocatalysts are better at breaking down pollutants than single-phase materials because more pollutants stick to the surface of the composite, and more active species can be generated for degradation reactions. 26To obtain porous and functionalized biochar, Ma et al. 80 mixed tobacco stem powder with K 2 CO 3 and low-density polyethylene in a certain mass ratio, followed by pyrolyzing at 900 °C under Ar atmosphere.The produced biochar with an extensive specic surface area and porosity enhanced the potential for adsorption and promoted the separation of photogenerated carriers.In Wu et al.'s study, 81 potassium hydroxide-modied algae-based biochar was successfully fabricated by calcining in the muffle furnace.Briey, 6 g of prepared biochar was combined with various proportions of KOH.Subsequently, the mixture was calcined in a muffle furnace at 800 °C.The prepared composite material (PEBC) with a weight ratio of 0.5 between biochar and KOH can achieve a maximum adsorption ability of 744.32 mg g −1 for sulfamethoxazole, which is far superior to other adsorbents.The test results of PEBCs indicate that the introduction of KOH in the preparation process of macroalgal biochar can effectively modulate the pore structure of the material, increase specic surface area, expand pore volume, and optimize pore size distribution, ultimately obtaining more adsorption sites and promoting pore lling.It is worth noting that the K element in the raw material helps to generate reducing gases (CO and H 2 ) during the calcination process, which not only accelerates the formation of the biochar's porous structure but also reduces high-valence metals to low-valence states. 82he adsorption mechanism of biochar on pollutants varies at different pyrolysis temperatures, including multilayer reaction, pore lling, mass transfer, p-p stacking interaction, and partition. 83,84At low temperatures, electrostatic interactions and hydrogen bonding are the main factors; at high temperatures, the pore lling and p-p stacking interaction dominates. 31pecically, the oxygen-containing functional groups (hydroxyl, carboxyl, carbonyl, and ester groups) on the surface of biochar can rapidly adsorb pollutants and transfer them to photocatalysts through p-p stacking, electrostatic attraction, and hydrophobic interactions. 85For example, Yu et al. 86 produced a ZnO/biochar composite photocatalyst that has strong electrostatic adsorption performance and can achieve a removal rate of 95.19% for MB.

Synthesis strategies for TiO 2biochar-based photocatalysts
TiO 2 is commonly employed as a photocatalyst due to its various advantageous properties, such as effective oxidizing capability, extraordinary chemical stability, and low toxicity. 49,87,88Biochar can be used as a supporting material and possesses excellent electrical conductivity, which has great applicability potential in the realm of photocatalytic oxidation. 11Combining TiO 2 with biochar is required to enhance the photocatalytic efficiency of semiconductors to remove organic contaminants.Synthesis strategies for TiO 2 -biochar-based materials have been widely investigated over the past few years.Generally, hydrothermal synthesis is a typical method to prepare TiO 2 nanoparticles.Regarding biochar, as mentioned above, a popular production process is biomass pyrolysis in oxygen-scarce environments.
Here, the preparation strategies of TiO 2 -biochar-based nanocomposites will be introduced by two sectors: in situ approaches and ex situ synthesis strategies.TiO 2 growth and loading biochar onto TiO 2 can be achieved simultaneously using in situ synthesis approaches.Typically, in situ approaches include solvothermal synthesis, sol-gel, chemical co-precipitation, and pyrolysis.An ex situ manufacturing approach attains the biochar combination by reworking the produced TiO 2 and biochar.Ex situ strategies mainly include impregnation-calcination, microwave treatment, plasma treatment, hydrothermal process, and bio-chemical method.TiO 2 -biochar-based photocatalysts synthesized in recent years using these methods are summarized in Table 1.

3.1
In situ synthesis 3.1.1Solvothermal synthesis.The solvothermal method, analogous to hydrothermal, is widely applied to produce catalyst nanoparticles.The solvothermal reaction occurs by dissolving the poorly soluble or insoluble substances to generate crystals under relatively high temperatures and pressure in a closed system. 18More importantly, the solvothermal method can yield well-crystalline nanomaterials by moderately tuning preparation conditions (such as temperature and residence time). 108Biochar is introduced to TiO 2 precursor to fabricate TiO 2 -biochar composites via the solvothermal method, an effective and typical in situ synthesis process.Using solvothermal, Wu et al. 103 successfully generated ower-like C, N co-doped, Ov-TiO 2 /C composites.In detail, the prepared bagasse-urea complex was mixed into tetrabutyl titanate acetic  The characterization result showed that the ower-like TiO 2 particles were successfully decorated on the surface of biochar.
In Fig. 1(c), the XRD pattern indicated that as-prepared owerlike TiO 2 particles show a classical anatase structure and a few weak peaks of the rutile phase.Therefore, the solvothermal method is efficient and convenient for preparing TiO 2 -biochar nanocomposites.
3.1.2Sol-gel.Sol-gel refers to the method by which organic metal compounds or inorganic salts are solidied through solution, sol, gel, and heat-treated to become oxides or other solid compounds. 9For example, synthesizing of TiO 2 -biochar nanocomposites, the biochar and titanate are stirred evenly in a specic solvent.Then, the pH of the solution is adjusted by nitric acid, and nally, the anti-solvent is added to form a sol.At the same time, the sample can be heated to accelerate the gelation process. 109Fig. 2 is the ow chart of Guo et al. 82 preparing biochar-nano TiO 2 cross-linked structure by sol-gel method.He rst added rice husk biochar to anhydrous ethanol containing tetrabutyl titanate and diluted nitric acid, then calcined the dried sample at 400 °C under N 2 protection.The heat treatment process can remove the organic compounds and accelerate the crystallization of sol-gel oxides. 110The specic surface area of biochar-nano TiO 2 is 779.87 m 2 g −1 , which is 6.5 times that of pure TiO 2 .This also directly proves that the composite of biochar and TiO 2 can effectively increase the specic surface area of the sample and further enhance the adsorption performance of the material for pollutants in the photocatalytic process.
3.1.3Chemical co-precipitation.Chemical co-precipitation is an essential and straightforward strategy for synthesizing metal oxide catalysts, which is performed by combining anions and cations in a solution and producing insoluble solid precipitation. 9Fazal et al. 93 synthesized biochar-TiO 2 composites using a wet precipitation approach.In brief, already prepared biochar (produced from macroalgae by pyrolysis) was dispersed in isopropanol, followed by the dropwise addition of titanium(IV) isopropoxide.The product was then elevated to 80 °C to exhaust the solvent.The sample was then placed in a muffle furnace heated to 400 °C with airow for 300 min.The SEM images exhibited that the spherical TiO 2 nanoparticles were evenly modied onto the biochar surface.Also, Herath et al. 111 successfully decorated Fe 2 TiO 5 nanoparticles on biochar (BC) using a chemical co-precipitation method and a heat treatment process.The XRD examination of Fe 2 TiO 5 and Fe 2 TiO 5 /BC corroborated the successful deposition of Fe 2 TiO 5 on the biochar surface.
3.1.4Pyrolysis.Mian et al. 106 fabricated N-doping TiO 2 -Fe 3 O 4 -biochar composites through one-step pyrolysis of FeCl 3 -Ti(OBu) 4 laden agar biomass, and the formation of N-doping was achieved via under NH 3 atmosphere.TiO 2 obtained by hydrolyzing tetrabutyl titanate is in situ grown onto biochar, followed by pyrolysis, a feasible synthetic path of TiO 2 -biochar composites.Luo et al. 112 added tetrabutyl titanate into distilled water solution and introduced as-prepared biochar into the system.The solution was vigorously stirred to ensure that biochar was well distributed.Subsequently, the obtained dried TiO 2 /biochar intermediates were activated at 500 °C for 120 min to form TiO 2 /biochar composites.
3.2 Ex situ synthesis 3.2.1 Impregnation-calcination.Recently, Li et al. 6 effectively utilized the soaking calcination to synthesize TiO 2 @BC ex situ, which differs from the in situ chemical co-precipitation method, where the already prepared TiO 2 was directly used.In addition, it can be adopted that TiO 2 and biochar are mixed by manual mechanical grinding, followed by calcining to prepare the composites.For instance, Lazarotto et al. 107 pyrolyzed the mixture of spent coffee biomass and TiO 2 via manual mechanical mixing to prepare biochar-TiO 2 composites.In the procedure, the biomass was converted into TiO 2 -impregnated biochar.Therefore, employing the impregnation-calcination method to prepare TiO 2 -biochar-based photocatalysts is attractive owing to its easy operation, cost-effectiveness, and reduced refuse production.
3.2.2Microwave treatment.Microwave treatment is an advanced technology for the ex situ synthesis of TiO 2 -biocharbased photocatalysts.Its prominent advantages are short heating treatment time and a safe operation process.The microalgae-based carbon dots (MCDs) were adorned on the outside of TiO 2 nanoparticles by Vu Nu et al. 94 via a straightforward microwave procedure.Due to the incorporation of MCDs, the photodegradation efficiency of the TiO 2 -MCDs composite was enhanced.In particular, as photosensitizers of electron acceptors, MCDs can acquire photogenerated electrons and boost the utilization of visible light.Notably, microwave power exerts a signicant inuence on the properties of synthesized composites. 49Oseghe and his coworker 96 fabricated pine cone-derived C-doped TiO 2 composites by microwave heating treatment at 600, 800, and 1000 W powers (labeled CT600, CT800, and CT1000, respectively).The studies showed that the BET surface area of composites decreased as microwave power increased.CT800 is better at breaking down tetracycline hydrochloride than other composites because it is more catalytic, has a lower rate of photogenerated carrier annihilation, and has more active species for the redox reaction.
3.2.3Plasma treatment.Dielectric barrier discharge (DBD), a promising technique for preparing and modifying photocatalysts, has been shown to broaden the utilization of TiO 2 in the non-UV region. 113Several reports have demonstrated that titanium undergoes a transformation of its crystalline and amorphous crystal structures aer hydrogen plasma etching, which can promote the utilization of visible light, reduce the binding of photoinduced carriers, and facilitate elemental doping, thus having an extended application in environmental protection. 114Recently, Tan et al. 98 successfully synthesized bamboo carbon/TiO 2 composites by H 2 plasma using the DBD method (Fig. 3).The results showed that the composite with a 5% bamboo carbon weight ratio has a more meso/ microporous structure, narrower band gap width, and lower electron-hole complexation rate and exhibits a good removal effect on formaldehyde.
3.2.4Other methods.Some new protocols for preparing TiO 2 -biochar-based photocatalysts, such as the hydrothermal process and bio-chemical method, have been developed for ex situ synthesis.Recently, Alomairy et al. 32 adopted the biochemical method for synthesizing biochar-modied TiO 2 / CoFe 2 O 4 core-shells.The primary process of integrating biochar with the TiO 2 /CoFe 2 O 4 core-shell is ultrasonic irradiation, which is considered a feasible and effective tactic.Characteristic results displayed that biochar is conducive to reducing the agglomeration of TiO 2 /CoFe 2 O 4 core-shell nanoparticles.Sánchez-Silva et al. 115 prepared hydrochar-TiO 2 composite (HC-TiO 2 ) via a hydrothermal process using Byrsonima crassifolia stones (BCS) as biomass materials.Photodegradation experiments showed that HC-TiO 2 can achieve 77% removal efficiency of crystal violet (20 mg L −1 ) within 90 min under UV light.Also, the reusability experiments indicated that the HC-TiO 2 composite has excellent stability for 5 times cycles.

Advantages of TiO 2 -biocharbased materials in photocatalytic degradation
We summarized some of the latest TiO 2 -biochar-based photocatalysts and TiO 2 -non-biochar-based photocatalysts in Table 2.The results indicated that TiO 2 -biochar-based photocatalysts generally exhibited superior photocatalytic performances and larger specic surface areas than TiO 2 -non-biochar-based photocatalysts.In detail, the advantages of TiO 2 -biocharbased materials in photocatalytic degradation can be embodied in ve aspects as follows:

Abundant reserves
Biomass contains nearly all of the living things on the planet and the biological material produced, processed, and expelled by them. 116Its broad existence and plentiful stocks ensure the creation of TiO 2 -biochar composites.Therefore, biochar has a conspicuous advantage in feedstock costs compared to other carbon materials and serves an essential function green energy.Moreover, TiO 2 nanomaterials that are cost-effective and have low toxicity can achieve a promising and long-term application in photocatalysis.Consequently, from the perspective of reserves, TiO 2 -biochar materials are recommended for cheap manufacturing and a green economy.

Large specic surface area
TiO 2 -biochar-based nanocomposites derived from biomass materials as carbon sources possess large specic surface areas because of biomass carbon's natural hierarchical porous microstructure. 117Specically, macropores promote rapid uid movement, whereas mesopores and micropores ensure complete interaction between contaminants and the photocatalyst surface.Also, biochar with a more porous structure makes TiO 2 -biochar-based photocatalysts better at capturing light by taking advantage of how light reects and scatters in the pores.More importantly, the surface active centers and exposed atoms of the materials increase with specic surface areas.These active sites and unsaturated atoms help stabilize the catalytic process's intermediates and reduce the activation barrier. 118In our previous study, 34 we reported a novel Au nanoparticles/TiO 2 nanorods/biochar (Au/TiO 2 /BCP) photocatalyst with a massive specic surface area of 1291.10 m 2 g −1 .Photodegradation results demonstrated that Au/TiO 2 /BCP reached a removal efficiency of 98.4% for degradation tetracycline, and per unit mass can remove 492 mg tetracycline.
Fig. 3 Sketch of the hydrogen dielectric barrier discharge modified system. 98herefore, the pollutant degradation rate is considerably improved using TiO 2 -biochar-based photocatalysts.

Boosting the adsorption of pollutants
Since the photocatalytic oxidation process happens on the exterior of the photocatalyst, a superior surface-based adsorption performance is necessary for the efficient destruction of contaminants. 119Biochar has more surface functional groups (such as hydroxyl and carboxyl groups) than activated carbon, which is one of the most pivotal factors inuencing adsorption efficacy. 120As a result, more pollutants in wastewater can be adsorbed and decomposed near the surface of photocatalysts, thereby achieving high photocatalytic efficiency, which is a crucial step in developing feasible applications of photocatalysts in wastewater treatment.

Strong interaction between biochar and TiO 2
Owing to the strong interaction between carbon materials and metal Ti atoms, biochar and TiO 2 can form a close contact structure.This improves the material's physical and chemical properties and the separation and migration of photoinduced charge, enhancing the lifetime of carriers and making them less likely to recombine. 121For instance, Zhai et al. 122 manifested that TiO 2 /bio-char had higher abilities of adsorption and decomposition for methylene blue than the bio-char, for which this composite existed a benign interaction between TiO 2 and bio-char and offered an enhanced electron transport efficiency.

Accelerating charge migration in the TiO 2 -biochar-based composites
In a TiO 2 -biochar composite photocatalysis system, biochar with appropriate conductivity can serve as an electron trap and a transit channel, promoting the separation and migration of photoinduced charge carrier pairs of TiO 2 and boosting the redox reaction.Furthermore, more reactive oxygen species can be generated by TiO 2 -biochar composites, thereby enhancing the photocatalytic activity. 123Hsu et al. 124 successfully deposited TiO 2 on the carbon nanospheres to construct carbon@titania yolk-shell nanostructures.The XRD results (Fig. 4a) indicated that TiO 2 hollow nanospheres prepared by sol-gel and calcination possess anatase structure.As shown in Fig. 4b, owing to the presence of carbon nanospheres, photoexcited electrons on the TiO 2 can be transferred, effectively inhibiting the recombination of electron-hole pairs.  5. Strategies for improving photocatalytic performance of TiO 2biochar-based photocatalysts

In terms of biochar
The improved strategies for biochar can be concluded in two aspects.For one thing, the preparation temperature of biochar was found to have signicant effects on its features and the performance of the TiO 2 -biochar photocatalyst.Xie et al. 133 fabricated biochar utilizing three distinct pyrolysis temperatures (300-700 °C).The results demonstrated that the TiO 2biochar prepared at 500 °C displayed the most effective purication of manufactured rainwater, as the stable tubular structure of the biochar facilitated TiO 2 nanoparticle attachment.For another, the surface modication of biochar is a vital point in achieving a uniform dispersion of nano-TiO 2 photocatalysts.Acid oxidization can erode the biochar surface, introduce functional groups, and expose more adsorption sites conducive to controllable supercial growth. 104,134The TiO 2 particles loaded on the surface of acid-treated biochar are smaller than those on untreated surfaces.This is because acid oxidation can effectively reduce the clumping of nanoparticles. 82Besides, biochar with a hierarchical porous structure can be achieved via activating an alkali, such as KOH. 101,135As a result, selecting suitable preparation conditions and functionalized treatment technology for biochar is an attractive tactic for enhancing its physical and chemical characteristics.On the other hand, controlling the conguration of TiO 2 nanocrystals at the microscopic level cannot merely accelerate the movement of charges but also facilitate mass delivery, resulting in an enhanced photocatalytic performance for the disintegration of organic toxins. 140Herein, Song et al. 141 synthesized TiO 2 nanomaterials with three different morphologies (nanocones, nanorods, and nanoparticles) using a Na 2 -EDTA-assisted hydrothermal technique (Fig. 6a), which were used as catalysts for the photoelectrocatalytic (PEC) removal of pollutants.The outcomes of the degradation experiment revealed that the nanocone-shaped TiO 2 (99.3%) had superior PEC performance compared to the nanorod-shaped TiO 2 (82.8%) and nanoparticle-shaped TiO 2 (62.7%) (Fig. 6b).This makes it easier to complete the mass transfer between catalysts with a conical array structure.Notably, the nanocone structure possesses a potent built-in electric eld that facilitates carrier separation and migration, a prerequisite for producing active oxides.In addition, Zhang et al. 142 synthesized well-aligned sub-10 nm TiO 2 nanowire arrays with adjustable corrugated structures by a distinctive mono micelle-directed assembly tactic (Fig. 7).Characterization results demonstrated that ultrathin corrugated TiO 2 nanowire arrays could separate photocarriers, transfer charges, and activate surface holes all at the same time.The remarkable optoelectronic behaviors are predominately determined by the exceedingly large surface area and rapid charge separation capability of the as-prepared, unusual TiO 2 arrangements.Consequently, it is desirable to nely manipulate the conguration of TiO 2 nanocrystals to ameliorate the photocatalytic efficiency of TiO 2 -biochar-based photocatalysts.

The synergistic effect of biochar and TiO 2
Finding the optimal amount of carbonaceous material to add to TiO 2 when making TiO 2 -biochar-based photocatalysts is important for biochar and TiO 2 to work together and for the photocatalytic activity to improve.Xie et al. 133 fabricated TiO 2biochar composites using three various TiO 2 -biochar mass proportion values by combining the necessary quantities of TiO 2 and biochar.The research results indicated that the purication effect of the TiO 2 /biochar composite on articial wastewater decreases with a decrease in mass ratio.This is because low mass ratio biochar blocks TiO 2 , weakens the transmission between light and TiO 2 , and loses photocatalytic efficiency. 143However, too high a quality ratio can backre, and the advantages of biochar materials cannot be fully utilized.Lu et al. 90 made several kinds of TiO 2 /biochar composite catalysts with varying biochar-to-Ti mass ratios.They designated it as CTx, where x (x = 0.1/1, 0.2/1, 0.5/1, 0.8/1, 1/1) indicated the biochar-to-Ti mass ratios.In the photocatalytic oxidation experiments of methyl orange, CT0.2/1 showed the best effect, with a decolorization rate of nearly 97% and a mineralization rate of nearly 83%.It indicates a synergistic effect between biochar and catalyst, and adding a proper amount of biochar can promote the photocatalytic process.
Besides that, TiO 2 -biochar nanocomposites with controllable porous microstructure and uniform morphology can be constructed via coupling with the template method, which contributes to the exposure of adequate activity sites for photocatalytic reactions.Wang et al. 144 fabricated hierarchical porous titanium dioxide/carbon nanocomposites (TiO 2 /C) by introducing ice and nanoscaled silica as so and hard templates for porosity modulating.The as-prepared TiO 2 /C nanocomposites exhibited more effective photocatalytic activity compared to their counterparts lacking templating, which can be assigned to the instructed porous structure and distinctive shape, which offer transfer avenues for electrolytes and photogenerated charge carriers, thereby enhancing the effectiveness of the degradation process.

Conclusions and future perspectives
In summary, TiO 2 , as an excellent photocatalyst, has been applied for almost half a century.To overcome the drawbacks of TiO 2 and further broaden its application range, biochar, a novel carbonaceous material with fascinating physicochemical properties and environmental benignancy, is employed to couple with TiO 2 in the eld of photocatalysis.Mainly, TiO 2 -biocharbased composites have been extensively used to photodegrade organic contaminants in wastewater.Outstanding photocatalytic performance is required for TiO 2 -biochar-based composites to be extensively applied and developed quickly.The reasoned design and manufacturing of TiO 2 -biochar-based photocatalysts provide a potent means of enhancing the efficiency of light capture and photogenerated carrier separation.Through substantial research, multiple techniques and strategies have been adopted to prepare and modify TiO 2 -biocharbased photocatalysts.In this review, we introduced the characteristics of biochar and summarized the synthesis approaches (including in situ and ex situ means), advantages, and strategies for optimizing the performance of TiO 2 -biochar-based photocatalysts.However, TiO 2 -biochar-based photocatalysts still need to be adequately explored.Although the distinctive advantages of TiO 2 -biochar composites should be approved, the properties of TiO 2 -biochar composites should be adequately researched, and the shortcomings of TiO 2 -biocharbased photocatalysts in the application of photodegradation pollutants should be rationally realized.These preliminary investigations are essential for the advancement of photocatalysts based on TiO 2 -biochar.In the following aspects, several signicant improvements in the research on TiO 2biochar-based photocatalysts can be accomplished.
First, as a dominating feature of biochar materials, the porous structure closely correlates with biochar's adsorption ability and photocatalytic performance.Meanwhile, the denite effect mechanisms have yet to be entirely explored.Thus, follow-up research concerning the mechanism of the porous structure of biochar remains to be done.
Second, regarding the combination of biochar and TiO 2 nanoparticles, the different loading proportions of TiO 2 on the surface and porous channels of biochar affecting photocatalytic activity have been extensively studied.Nonetheless, the specic interaction of TiO 2 load proportions and the as-synthesized composites of physical and chemical properties needs to be more explicit.In the future, the quantitative structure-activity relationship (QSAR) of TiO 2 load proportions and structural/ optical features (e.g., BET, pore volume, pore type, and light absorption ability) of TiO 2 -biochar-based composites is desirable to investigate thoroughly, which has a signicant effect on the understanding and reasonable design of high-performance TiO 2 -biochar-based photocatalysts.Third, the computational calculation for the pathways of TiO 2 -biochar-based photocatalysts degradation pollutants is an effective means to investigate the decomposition process of pollutants.Unfortunately, limited by the fact that it is challenging to build the theoretical calculation model due to the complex structure of biochar, the theoretical calculation of photocatalytic degradation pollutants by TiO 2 -biochar-based photocatalysts has yet to be developed comprehensively.More efforts on the theoretical calculation of TiO 2 -biochar-based photocatalysts are necessary, including the adsorption mechanism between photocatalysts and pollutants, electrons transfer routes, and the minute decomposition pathway of contaminants.
Fourth, although we summarized many different methods for preparing TiO 2 -biochar-based photocatalysts in this review, large-scale manufacture still needs to be improved in real-world applications.Also, the high production costs always limit the industrialization process of TiO 2 -biochar-based photocatalysts.In order to decrease the costs of photocatalytic elimination of organic pollutants in water further using TiO 2 -biochar-based photocatalysts, some appropriate strategies should be extensively investigated.
Fih, most studies of TiO 2 -biochar-based photocatalysts for the degradation of pollutants were implemented under UV light, although a favorable removal efficiency can be reached.Engineering strategies of TiO 2 -biochar-based photocatalysts should be explored to tune intrinsic physicochemical properties and to expand the response of visible light.Besides, developing multi-functional and highly active TiO 2 -biocharbased photocatalysts is needed to degrade the wastewater that consists of various persistent pollutants.Therefore, further investigation of TiO 2 -biochar-based photocatalysts can focus on adjusting properties and regeneration to adapt to practical applications.
It is worth exploring these aspects we discussed, for which TiO 2 -biochar-based materials have a promising future for photocatalytic degradation.Reasonable design and superior performance for TiO 2 -biochar-based photocatalysts can conspicuously boost the development of wastewater treatment and energy conversion.This review will present a comprehensive cognizance of TiO 2 -biochar-based photocatalysts.In the future, broader application elds of TiO 2biochar-based materials could be realized through more research, such as applying them in solar cells, adsorption, and supercapacitors.
acid solution, and then the mixed solution initiated a solvothermal reaction in an autoclave at 180 °C for 720 min.The nal product was sintered at 450 °C in N 2 atmosphere for 120 min.Fig.1(a) and (b) is the SEM of the obtained samples.

5. 2
In terms of TiO 2The exposed facets of nanocrystals can easily affect chemical reactions on the external layers of photocatalysts during the photodegradation process.More importantly, different crystal facets present different surface atomic arrangements and crystallographic orientations, which determine the spatial charge separation between facets and reactive sites and diverse molecule absorption abilities.136,137Qu et al.138 investigated the photocatalytic water splitting performance of anatase TiO 2 with predominant {001} facets and {111} facets, showing that the {001} facets displayed higher activity.The substantial amount of surface undercoordinated Ti atoms and the surface atoms' strained conguration are advantageous for water adsorption and dissociation.Besides, as shown in Fig.5, in situ photochemical probing reactions have corroborated that more nucleation sites exist on the {001} facets of anatase TiO 2 .In addition, utilizing the strong oxidation ability of TiO 2 (001) facets, Li et al.139 successfully designed a MoS 2 /Au/TiO 2 heterogeneous photocatalyst with highly-exposed (001) facets of TiO 2 .Notably, the photogenerated holes in this composite are primarily on the large TiO 2 (001) facets.This means that more oxidation sites are exposed, which speeds up the photocatalytic removal process.Thus, to boost the photocatalytic capacity of TiO 2 -biochar-based photocatalysts, it is crucial to tune the exposed facets of TiO 2 nanocrystals during the manufacture of TiO 2 -biochar-based composites.

Fig. 5
Fig. 5 Photo-deposition of Pt on different facets after one minute of illumination.(a and b) HAADF images of Pt deposited on {001} facets.(c and d) HAADF images of Pt deposited on {111} facets.138

138
Fig. 5 Photo-deposition of Pt on different facets after one minute of illumination.(a and b) HAADF images of Pt deposited on {001} facets.(c and d) HAADF images of Pt deposited on {111} facets.138

Fig. 6
Fig. 6 (a) Synthesis scheme of TiO 2 catalysts with different structures by a Na 2 EDTA-assisted hydrothermal approach.(b) The percentage of 4-CP degradation. 141

Fig. 7
Fig.7Schematic illustration of the formation process for the ultrathin corrugated TiO 2 nanowire arrays synthesized via the monomicelledirected assembly approach.142

Table 1
Summary of the preparation methods and their photocatalytic degradation activity of recently reported TiO 2 -biochar-based photocatalysts